Rapid depletion and environmental aspects of fossil fuels, coupled with highly volatile market prices have made the quest for alternative fuels a top-priority. Over the past two decades, several alternatives to liquid fuels have been investigated, which have the potential either to completely replace the petroleum-derived fuels in some sectors or be blended with petroleum-based fuels in variable proportions, without modifying the vehicle engines and existing infrastructure.

Now, there is also competition for production between renewable fuels. Ethanol plants can produce fuel ethanol but they can also be retrofitted or become the host facility to produce butanol. Butanol differs from ethanol in composition, physical characteristics and economics. We present the case for renewable butanol production.

Made from petrochemicals, Isobutanol has a variety of uses as a solvent and chemical building block. In the past few years more technologies have been used to create Isobutanol from renewable resources. Financially, creating Isobutanol from renewable feedstocks yields a product that cannot compete with the existing Isobutanol production techniques. Therefore the production of renewable Isobutanol has been severely limited to small-scale production.

However, sometimes a renewable product can prove to be an extremely effective fuel[i]. In the United States, Renewable Isobutanol qualifies as an Advanced Biofuel and has an approved pathway for production from many feedstocks. The Environmental Protection Agency (EPA) rewards Isobutanol with renewable energy credits through its Renewable Energy Standard. In fact, the EPA assigns renewable credits (RINS[ii]) on the basis of energy content, and Isobutanol has higher energy content (1.3 RINS per gallon) than ethanol (1.0 RINS per gallon).

Of course, Isobutanol production comes at a cost. The cost offset of RINS has not been above 35% of Isobutanol’s intrinsic value. Even with the RIN value considered, the product is still more than twice the price of ethanol, on average[iii]. Consequently, Isobutanol has not generally been used as a substitute for ethanol in most applications. However, recently gasoline makers have begun to use Isobutanol to serve a growing market for alternatives to fuels containing ethanol. But do the positives outweigh the negatives?

Let’s go back to the differences between ethanol and Isobutanol in Table I. Each provides a source of oxygen. Ethanol used at a 10% volume blend equates to 3.7% oxygen by weight in gasoline. The EPA has approved the use of Isobutanol at a blend level of 12.5% by volume, which results in 2.7% oxygen by weight in gasoline. So, gasoline-containing Isobutanol has less oxygen. Oxygen has zero BTU by definition, so the Isobutanol-blended gasoline will have more BTUs – more power – than a 10% ethanol blended gasoline. More power is good and positively impacts the value of the gasoline. Isobutanol-blended gasoline also produces a more satisfactory performance than a gasoline containing ethanol.

Table I Comparison of Properties of Different Fuels

Fuel

Gasoline

n-Butanol

Iso-butanol

Ethanol

Methanol

Energy density (MJ l−1)

32

29

29

19.6

16

Vapor pressure (kPa) at 20 °C

0.7–207

0.53

1.17

7.58

12.8

Vapor pressure of mixture with gasoline (kPa)

53.8–103.4

44.1

46.9

138

800

Air:fuel ratio

14.6

11.2

—

9.0

6.5

Heat of vaporization (MJ kg−1)

0.36

0.43

—

0.92

1.16

Research octane number

91–99

96

—

129

129–134

Motor octane number

81–89

78

112

102

97–104

Cetane number

—

—

—

54

Freezing temperature (°C)

Less than −60

−89.5

−108

−114.5

−97.6

Hygroscopicity

Low

Low

Low

High

High

Compatibility with existing infrastructure

Yes

Yes

Yes

No

No

The EPA limits the vapor pressure of gasoline to reduce emissions of hydrocarbons. Ethanol, by itself, has a fairly low vapor pressure, but when combined with gasoline it actually polarizes away from the gasoline and increases the gasoline’s vapor pressure. Isobutanol also has a low vapor pressure, but as opposed to ethanol it blends well with gasoline, depressing its vapor pressure. From a gasoline maker’s viewpoint, a less costly gasoline can be used to make an Isobutanol-blended gasoline. The lower cost of the gasoline component is a positive impact on price.

To summarize, Isobutanol costs more than ethanol (negative) but has more power and can be used in less costly gasoline (positive). Unfortunately, the positives do not yet outweigh the negatives, so an Isobutanol-blended gasoline cannot be cost competitive with ethanol-blended gasoline. This is why almost all refiners dropped consideration of an Isobutanol-blended gasoline.

One oil company took an alternative view. Gulf Racing Fuels is responsible for the off-road fuels of Gulf Oil. They specialize in boutique fuels to meet the needs for high performance and specialty engines. Most importantly, Gulf does not use ethanol. Gulf knew that there was an entire class of consumers who only wanted an ethanol-free fuel. They also knew this market segment had very high price elasticity. Stated more simply, they would pay more for an ethanol-free fuel.

In 2014, the first commercially available ethanol-free EPA-approved gasoline was created by Gulf Racing Fuels under the brand names Gulf Marine, Gulf ATV, and Gulf Off Road; Table II. In 2015, the National Marine Manufacturers Association endorsed gasoline blended with Isobutanol over all ethanol-blended gasoline.

Now Gulf is in the process of transferring the intellectual property of the ethanol-free gasoline to Gulf-branded marinas and gas stations. Other oil companies will be joining in the market as the fuel can be produced and sold profitably in commercial volumes.

Table II Gulf Ethanol Free Gasoline

Trade Names

Gulf Marine, Gulf Off-Road, Gulf Race Fuel

Octane

93 and 100 (R+M/2)

Isobutanol

12.5 by volume

RVP

6.5 psi

Price

$4.00 – $8.00/gallon

The big question is: will there be enough volume of renewable Isobutanol? The answer is no. In the short term, oil companies will source additional Isobutanol from petrochemical manufacturers. In the long term, there will be a significant need for more renewable Isobutanol production. Where will the new supplies of Isobutanol be made? To get to that answer we need to know how renewable Isobutanol can be made.

Means of production

Starting from petroleum sources, butanol is typically produced either by the Oxo-process utilizing propylene with hydrogen and carbon monoxide (usually in the form of synthesis gas) [(i) and (ii)] over an expensive rhodium catalyst, or the Aldol-process starting from acetaldehyde.

The four general variants of the Oxo process in use today are all catalyzed by expensive rhodium, but use different ligand systems. Each variant produces its own n/iso ratio ranging from 1.6/1 to 30/1. Older catalytic systems could produce a 1/1 ratio of the two isomers. Despite the extremely expensive catalyst system – the process has been adopted to meet 95% of world butyraldehyde demand (With a global consumption >7×106 t/a, about 6 billion kilograms are produced annually by this route alone).

Contaminants such as hydrogen sulfide and carbonyl sulfide (often found in commercial propylene and synthesis gas streams), and organic chlorides often seen in propylene, or dienes were definite catalyst poisons. To avoid catalyst deactivation by trace components present with propylene – The impurity guard beds and other purification plant that Davy Process Technology developed for commercial feedstocks ultimately featured in the design of commercial LP OxoSM plants.

The Green Pathway

The second, green-pathway is the ABE-fermentation process (Acetone, Butanol, Ethanol) that was pioneered by Chaim Weizmann during World War I. During 1950s the ABE fermentation process was not competitive with natural petroleum sources and hence ignored in the US and European industry. However, some production processes continued in China, Russia and South Africa until the early 1980s. This has changed dramatically in recent years. For example, China has lead efforts to re-commercialize the ABE fermentation process. With investments over $200 million towards installing 0.21 million tons per annum of solvent capacity with plans to expand it to 1 million tons per annum.

A number of biotechnology ventures with proprietary approaches to microbial strain selection and engineering, feedstock handling, bioreactor management, and product separation have emerged as a new generation of butanol producers since early 2000. Depending on the niche one may produce either n/iso butanol. Most of these operational plants operate in a semi-continuous mode to minimize downtime, and the lag phase wherein one fermentation cycle can take almost 21 days. In addition, the plants are located either immediately next to the ethanol plants or retrofitted to reduce utility and operating costs.

Co-located operations tend to share effluent treatment facilities based on anaerobic digestion (AD). Biogas, produced from the AD process, can be used to generate power and heat. The recovery of hydrogen from the fermentation exhaust gas (typically 1/10 of mass of butanol produced) gains an additional value to the process.

The process limits

The mass and energy yields determine the absolute, theoretical limits to the process economics. The theoretical mass and energy yield are in the vicinity of 35% and 90% respectively, calculated on the basis of product ratio and energy combustions in the fermentation. The substrate costs, account for almost 60-65% of the total production cost, and play a vital role in the economics. In order to reduce substrate costs in typical bulk-chemical fermentation processes, the fermentation plant must be able to use a variety of substrates including those of low-grade substrates such as lignocellulosic biomass, waste material from food and feed industries, and waste from agricultural industries.

Typically, butanol production costs are at $1.56 gal−1 based on $1.80 per bushel corn feedstock cost. If the feedstock were raised to $3.35 per bushel, the estimated butanol cost would be $2.10 gal−1. Whilst the economy of fermentation route is more sensitive to the price of the substrate than butanol yield; it is estimated that the process will not be feasible if the yields are less than 25% w/w.

Strain improvement

Strain improvement is an effective method to improve the yield; however it has a lower influence on the process economics as compared to other factors (mainly cost of substrate, and product recovery costs). With an improved strain tolerance for higher butanol concentrations and an increase in volumetric productivity by about 50%, the production costs for bio-butanol would be similar to the production costs for petroleum derived butanol. Moreover, if the final solvent concentration can be increased by up to 25–30 g l−1 and if the fermentation time of the batch fermentation around 60 hours can be maintained, the ABE fermentation should be industrially feasible. In order to reach economic viability, reductions in both conversion cost-intensity and in recovery cost-intensity are required.

The process economics can further be increased with in situ product removal techniques. The use of the two-stage continuous solvent-producing cultures with the immobilized biomass or biomass-retention achieves higher solvent productivities with improved substrate consumption and reduced the solvent toxicity. In addition, batch fermentation processes have been found less economical than continuous processes as they demand additional sterilization steps for vessels, pipes and valves. However, cross contamination issues with continuous processes pose their inherent challenges.

The product market

The product market is another significant parameter in the economics of butanol fermentation. The present sizeable market of ABE fermentation is still fascinating industrialists and researchers and it is expected that after adapting to bio-butanol as a fuel, the market demands will be high. By-products including acetone, ethanol, H2, and CO2 can also contribute significantly to the butanol production economics. It should be noted that butanol production itself is a cost-intensive methodology and recovery technology after the fermentation results in a higher cost-intensive process. Improvement in butanol tolerance of strain and selective in situ solvent removal can enhance the fermentation time, productivity and other economic feasibility aspects of the process, which can be materialized in the economic feasibility of process. New recovery processes such as gas stripping, combined with advanced membrane separation technologies may contribute to the economic feasibility in future.

Investment costs largely affect the economic assessment of the fermentation route. The large capital investment before the actual production starts has a huge impact on the overall economics of the process. Besides, investment costs directly increase the total costs both as depreciation and for financing (interest and repayments). Hence, major reductions in the investment costs and/or delaying investments especially until the production starts will help to improve the process economics.

One of the biggest challenges in the future will be the scaling-up process in a cost-effective manner. The petroleum industry’s demand for Isobutanol will consume the entire fuel grade production that will qualify for the EPA Renewable Fuel Standard (RFS). Economics of production will dictate the number of plants created and total output. What is clear from our perspective is that Butanol will have a place in the fuel markets for the USA.

About the Authors

Jess Hewitt is the RIN and biofuels off-take expert for Lee Enterprises Consulting, Inc, the world’s largest bioenergy consulting group. He received his B.S. in Economics from the University of Houston in 1980. He is a fuel expert especially in the blending of renewable fuels into petroleum products and marketing of the blended products. Currently he serves as consultant for Lee Enterprises, Chairman of Gulf Hydrocarbon, Director of Syndiesel and President of Hyperfuels LLC dba Gulf Racing Fuels. Jess has over 30 years’ experience in the energy industry. He is a former member of the National Biodiesel Board and the past Chairman of its Marketing Committee. He served as President of the Biodiesel Coalition of Texas and on the advisory board for the New York Mercantile Exchange (NYMEX). Jess also oversees isobutanol projects – marketing, fuels registration for EPA, RFS2 registration, along with manufacturing and importer registration with the EPA and IRS.

Dr Kapil S. Lokare is the Director of European Operations for Lee Enterprises Consulting, the world’s largest bioenergy consulting group. He received his Ph.D. in chemistry from Michigan State University in the United States. In addition to working with bio-based start-ups and creating business insights towards sustainable markets, he has held positions at various globally recognized institutions in the U.S.A., The Netherlands, Australia and Germany. With a strong network of clients based in South/North America, Europe, Southeast Asia and the Oceania, Kapil has helped launch new bio-based startups and currently resides in Berlin, Germany and oversees the execution and operation of 2, 60KLPD ethanol distilleries.

[i] Fuel – in this case we consider Isobutanol used as an oxygenate in gasoline, other fuel uses are ignored.